1. Field of the Invention
The present invention relates to step-up switching power supply devices (chopper power supply devices) with synchronous rectification that produce an output voltage by stepping up an input voltage, and to electronic devices provided with such step-up switching power supply devices.
2. Description of Related Art
As a conventional technology related to the present invention, a step-up switching power supply device disclosed in JP-A-2006-304500 (hereinafter “Patent Document 1”) has already been proposed by the applicant of the present invention. As shown in FIG. 4, this step-up switching power supply device includes: an input terminal Ta to which an input voltage Vin is applied via an external inductor Lex; an output terminal Tb from which an output voltage Vout to a load is outputted; an output transistor N1 connected between the input terminal Ta and a predetermined reference voltage node; a first P-channel field-effect transistor P1 connected between the input terminal Ta and the output terminal Tb; a second P-channel field-effect transistor P2 connected between the output terminal Tb and the back gate of the first P-channel field-effect transistor P1; and a switching control portion CTRL performing switching control on the output transistor N1 and the first and second P-channel field-effect transistors P1 and P2. When the input voltage Vin is stepped up to obtain the output voltage Vout, the switching control portion CTRL performs switching control on the output transistor N1 and the first P-channel field-effect transistor P1 complementarily while maintaining the second P-channel field-effect transistor P2 always in an on state; when the driving of the device is stopped, the switching control portion CTRL controls the output transistor N1 and the first and second P-channel field-effect transistors P1 and P2 so as to be brought into an off state.
As described in detail in Patent Document 1, with the conventional step-up switching power supply device described above, the current path along which a current flows via a body diode BD1 attached to the transistor P1 can be cut off with the transistor P2 when stopping the driving of the device.
That is, with a configuration in which the back gate and the source of the transistor P1 are simply connected to each other to give the transistor P1 better switching characteristics, a current undesirably flows through a current path from the inductor Lex to the load via the body diode BD1 while the driving of the switching power supply device is stopped. By contrast, with a configuration in which the transistor P2 is provided on the current path from the inductor Lex to the load, and both the transistors P1 and P2 are brought into an off state when the driving of the switching power supply device is stopped, it is possible to prevent a leakage current from flowing from the inductor Lex to the load by cutting off the current path. Thus, with the conventional step-up switching power supply device described above, it is possible to appropriately cut off the current path from the node to which the input voltage Vin is applied to the load according to circumstances.
Incidentally, as increasingly high efficiencies and high output voltages have been sought in step-up switching power supply devices, increased current capacities of the transistors P1 and P2 provided on the current path from the inductor Lex to the load have been sought after. However, if the transistors P1 and P2 are simply increased in size, problems may arise, such as an abrupt increase in coil current Icoil at startup of the device, as shown in FIG. 5, leading to an unstable rising behavior of the output voltage Vout.
FIG. 5 shows, from top to bottom, the voltage waveforms of a power-off signal Soff, a gate signal Sx, a gate signal Sy, a gate signal Sz, a switching voltage Vsw, and an output voltage Vout, and the current waveform of a coil current Icoil.
One factor responsible for an increase in coil current Icoil at startup of the device (immediately after the power-off signal Soff transitions to a low level), as shown in FIG. 5, is the presence of a leakage current flowing through the transistors P1 and P2.
In order to ensure that the transistor P1 is turned off during the off period of the transistor P1 (a high level period of the gate signal Sy), it is necessary to apply a gate signal Sy having a higher voltage level than that of the input voltage Vin. However, since the gate signal Sy is produced from the output voltage Vout, even during a period when the transistor P1 should be in an off state, it cannot be turned all the way off until the time when the output voltage Vout reaches the input voltage Vin. As a result, a leakage current flows to the output terminal Tb side via the transistor P1, leading to an undesirable increase in coil current Icoil. On the other hand, during the on period of the transistor P1 (a low level period of the gate signal Sy), a current flows to the output terminal Tb side via the transistor P1 until the time when the output voltage Vout reaches the input voltage Vin, resulting in an undesirable increase in coil current Icoil.
In addition, since the transistor P2 connected between the back gate and the source of the transistor P1 is maintained always in an on state from startup of the device, a leakage current continues to flow to the output terminal Tb side via the body diode BD1 and the transistor P2 until the time when the output voltage Vout reaches the input voltage Vin, leading to an undesirable increase in coil current Icoil.
In particular, during the on period of the transistor N1, due to a passage not only of an unintended leakage current through the aforementioned transistors P1 and P2 but also of a current through the transistor N1, the coil current Icoil increases in the form of a peak, leading to an unstable rising behavior of the output voltage Vout.
Certainly, some conventional step-up switching power supply devices have an overcurrent protection capability. However, this overcurrent protection capability is designed only to monitor a current flowing through the transistor N1. As a result, as long as the monitored current is within the normal range, no matter how large an inrush current flowing through the transistors P1 and P2 is, it is impossible to reduce the inrush current, and hence prevent the above-described increase in coil current Icoil.
Furthermore, as shown in FIG. 5, the coil current Icoil shows a transitional increase when a step-up operation of the output voltage Vout is started as a result of the output voltage Vout having reached the input voltage Vin.